The present invention relates to a touch panel adapted to be mounted on a display device such as CRT and detect a coordinate at a position of depression by a finger, a pen, etc.
Generally, the touch panel is formed by laminating transparent upper and lower electrodes with a spacer such as a dot spacer interposed therebetween, and supporting the lower electrode to a transparent support plate. The touch panel thus formed is mounted on a display surface of a display device such as CRT or LCD (liquid crystal display), and is depressed at a predetermined position by means of a pen, a finger, etc. as viewed by an operator to detect a coordinate at such a depressed position, thus providing a coordinate input device.
In such a touch panel as mentioned above, it is known that a filter permitting transmitting of a certain part of spectrum from the display device as a light source and absorbing the other part is provided so as to improve a contrast at a display portion of the display device and adjust a color tone.
Such a conventional filter is formed by uniformly dispersing pigment or dyestuff made of organic or inorganic materisls in a transparent acrylic resin, and the filter is coated on a single surface or both surfaces of the upper electrode and the lower electrode. Alternatively, in the case that the transparent support plate is formed of a transparent synthetic resin such as acrylic resin, pigment or dyestuff as mentioned above is kneaded in the transparent support plate to integrally form the filter with the support plate. As a result, the steps of manufacturing the touch panel with such a filter as mentioned above are complicated to cause an increase in cost.
Further, in the prior art touch panel as mentioned above, an air gap is defined by the dot spacer between the upper electrode and the lower electrode. Accordingly, there occurs reflection or refraction of light at an air layer formed by the air gap, causing a reduction in transmission rate of the touch panel as a whole. Additionally, the outline of the dot spacer is readily visualized to generate a problem in visibility.
Furthermore, as the dot spacer is fixed on either of the electrodes, there exists a non-contact area of both the electrodes, that is, a dead zone of the input device, causing incapability of minute coordinate detection. Particularly in the case of forming the dot spacer by a general printing technique, a diameter of the spacer tends to be greater at a bottom surface than a height thereof in relation to the viscosity of the spacer. Therefore, the existence of the dead zone as mentioned above cannot be ignored.
Other documents disclosing a touch panel and a membrane switch are co-pending U.S. patent application Ser. Nos. 07/187,263, filed Apr. 28, 1988, entitled "Membrane Switch" and 07/180,294, filed Apr. 12, 1988, entitled "Touch Panel".
In another conventional touch panel as disclosed in Japanese Utility Model Laid-Open Publication No. 60-112844, a transparent insulating liquid acting as a spacer is enclosed between the upper electrode and the lower electrode. When the touch panel is depressed from the upper electrode side, the insulating liquid at a depressed portion is moved aside to allow the contact between both the electrodes, while when the depression force is removed, both the electrodes are maintained separate from each other owing to the fluidity of the insulating liquid. Thus, as there are absent the air layer and the dot spacer as mentioned previously between both the electrodes, the visibility may be improved, and the resolution may be also improved because of no dead zone.
Although such a touch panel utilizing the insulating fluid as a spacer exhibits a good function regarding the prevention of reduction in visibility and the improvement in resolution, there still remains a problem. That is, the insulating liquid has a good fluidity in response to a depression force, but it is inferior in restorability when removing the depression force. Accordingly, there is a possibility of both the electrodes erroneously contacting with each other during a long service life. Further, as a long restoring time is needed, operation feeling is rendered unsatisfactory.